This invention relates to interframe coding apparatus for the transmission of a video signal in the form of a digital signal, which compresses the frequency bandwidth of the signal by utilizing a high interframe correlation of the signal.
The PCM system is the simplest method of converting a video signal into a digital code, but requires a high bit rate of 64Mb/s for a monochrome television signal of 4MHz and a high bit rate above 80Mb/s for a color television signal of 4.2KHz, resulting in high transmission cost. To avoid this, there have been developed the DPCM system and .DELTA.M system utilizing an inframe correlation but their bit rate is merely one-half of that of the PCM system. Also, the interframe coding system utilizing an interframe correlation has been developed for further reduction of the bit rate.
With the interframe coding system, only information of a change in a frame with respect to its predecessor is transmitted, and since this information does not occur uniformly, it is once stored in a buffer memory, and thence delivered out to a transmission line at a constant bit rate. Accordingly, in the case of a rapid change in the frame, the amount of information to be transmitted is increased, which results in an overflow from the buffer memory to cause the picture quality of a reproduced picture to be appreciably impaired. For the prevention of occurrence of the overflow, only a little reduction of the bit rate is possible. On the other hand, a buffer memory having a sufficient capacity to prevent such an overflow may be used, but is not preferred in the case of a video telephone because of too much a delay of the picture signal relative to the voice signal. In view of this, the so-called FRODEC system has been proposed, as described, for example, in an article "Transmitting Television as Clusters of Frame-to-Frame Difference" by J. C. Candy et al. on Bell System Technical Journal Vol. 50, No. 6 . According to this system, the difference between the input signal and a signal from a frame memory, which exceeds a threshold value, is encoded and supplied to the buffer memory, and as the buffer memory is approaching its full capacity, the threshold value is increased to reduce the signal which is supplied to the buffer memory. With such a system, the bit rate can be reduced to about one-tenth of that of the PCM signal. However, transmission even at such a bit rate is still appreciably costly as compared with voice transmission. Further reduction of the bit rate by the FRODEC system can be achieved by greatly increasing the threshold value, but this considerably increases the generation of quantizing noise, resulting in degraded picture quality of the reproduced picture.
This invention has for its object to provide interframe coding apparatus which is capable of further reduction of the bit rate without substantial degradation of the picture quality of the reproduced picture.